It is known to use an identifier, typically carried by a user of a vehicle, to detect the approach of the user and then automatically command at least one function of the vehicle (such as unlocking of the doors of the vehicle).
It is provided, for example, for this purpose, for a low-frequency emitter module of the vehicle to generate an electromagnetic signal (with a frequency of typically less than 150 kHz) having a given range.
When the identifier enters into the range of the signal emitted by the vehicle, it emits a response packet so as to indicate its presence, and communication is then established between the identifier and the vehicle, thereby normally making it possible (in particular if the identifier indeed corresponds to the vehicle in question) to bring about the implementation of the abovementioned function.
In addition, the identifier carries out a measurement of the strength of the signal generated by the low-frequency emitter module, thereby enabling it to accurately evaluate the distance separating the identifier and the vehicle (using a technique commonly called RSSI, for received signal strength indication).
This distance is generally transmitted to the vehicle via the communication that is established as indicated above, thereby enabling the vehicle to know the distance from the user, and to implement certain functions only when the user is close enough to the vehicle.
In general, it is sought to increase the range of such a system in order to detect the arrival of a user as early as possible and to evaluate the distance separating the vehicle and the identifier as soon as possible.
However, in the system described above, such an increase in range requires an increase in the size (and therefore in the weight and in the cost) of the low-frequency emitter module, which quickly offsets the benefit that this solution has in terms of accuracy.